This invention relates to gauges for measuring the inside diameter of bores and more particularly to helically wound bore gauges.
A typical nuclear steam generator comprises a vertically oriented shell, the plurality of U-shaped tubes disposed in a shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite the U-like curvature, and a dividing plate that cooperates with the tube sheet, forming a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle. The primary fluid, having been heated by circulation through the nuclear reactor core, enters the steam generator through the primary fluid inlet plenum. From the primary fluid inlet plenum, the primary fluid flows upwardly through first openings in the U-tubes near the tube sheet which supports the tubes, through the U-tube curvature, downwardly through second openings in the U-tubes near the tube sheet and into the primary fluid outlet plenum. At the same time, a secondary fluid, known as feedwater, is circulated around the U-tubes in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes, causing a portion of the secondary fluid to be converted to steam. Since the primary fluid contains radioactive particles and is isolated from the secondary fluid by the U-tube walls and the tube sheet, it is important that the U-tubes and the tube sheet be maintained defect-free so that no breaks will occur in the U-tubes or in the welds between the U-tubes and the tube sheet, thus preventing contamination of the secondary fluid by the primary fluid.
Occasionally, it is necessary to either inspect or repair the U-tubes or the tube sheet welds by way of access through the primary fluid inlet and outlet plenum. For this purpose, manways are provided in the vertical shell so that working personnel may enter the inlet and outlet plena to perform operations on the U-tubes and the tube sheet. However, since the primary fluid, which is generally water, contains radioactive particles, the inlet and the outlet plena become radioactive which thereby limits the time that working personnel may be present therein. Accordingly, it would be advantageous to be able to perform the inspection operations on the U-tubes as quickly as possible so as to minimize the time that working personnel must be present in the nuclear steam generator.
One type of inspection operation that is performed occasionally on the nuclear steam generators is inspection of the U-tubes to determine if any of the U-tubes have become dented. Denting of the U-tube is thought to be caused by several mechanisms, one of which is corrosion build-up on the outside of the tubes which dents the tubes and causes a constriction in the inside surface. Denting of the U-tubes may lead to cracks in the U-tubes at the dented sight which thereby allows the primary fluid which contains radioactive particles to leak into the secondary fluid through the U-tube walls. This, of course, is an undesirable result which should be prevented.
In order to determine if any of the U-tubes have become dented, occasionally working personnel enter the nuclear steam generator and inspect the U-tubes. Inspection of the U-tubes generally comprises measurement of the internal diameters of the tubes to determine if the tube is dented. There are several mechanisms known in the art that can be used to measure the internal diameter of the U-tubes, however, none of them is completely satisfactory for all purposes. For example, most of the internal diameter gauges suffer from the problem of insufficient durability and need to be replaced often.
Therefore, what is needed is a bore gauge that is capable of inspecting the internal surface of a tube in a nuclear steam generator in a quick and reliable fashion.